For the purpose of optical data transmission an optical signal may be modulated in its phase and/or amplitude. An optical carrier signal, which has a respective wavelength bandwidth, may be modulated in accordance with a specific modulation scheme having a respective order. Due to the increase of data traffic, modulation schemes or modulation formats of higher order may be used in order to increase the amount of transported information for a given carrier signal. With the introduction of coherent data transmission using coherent detection at a receiving side, a variety of modulation schemes becomes available. The optical carrier signal modulated at the transmitting side is detected at a receiving side in a coherent reception scheme using a local oscillator signal that corresponds to the wavelength of the received optical signal. Other multiplexing methods, such as polarisation multiplexing, may be deployed for furthermore increasing the overall data rate.
An advantage of coherent data transmission is the possibility of creating optical channels consisting of multiple carrier signals, such that a narrow spectral occupancy is achieved. In such a technology called flexgrid, the spacing of the optical channel may consist of spectral slots having a bandwidth of 12.5 GHz, wherein an overall slot of for example 50 GHz may be reserved for one single optical carrier signal exploiting the bandwidth of this overall slot. In other words, according to this technology a bandwidth varied within steps of 12.5 GHz may be allocated by an optical carrier signal.
It is a drawback of the so called flex grid solution, that this solution needs filtering devices at the optical network nodes that are compliant with the specific spacing of this specific grid. For example, different works in the literature have demonstrated that filters for bandwidths narrower than 35 GHz may not show satisfying performance in terms of insertion loss and profile sharpness. Therefore, applying an individual optical filter to an individual carrier signal of a bandwidth smaller than 35 GHz may be cumbersome.
FIG. 1 shows a power spectral density PSD of two carrier signals C1, C2 over the wavelength WL for a slot spacing given by slots S1, . . . , S12. The bandwidth of the different slots SL is equal for all slots. The two signals C1 and C2 may form a so called super-channel signal. A super-channel signal is a signal that is transmitted in an optical network from a same starting and transmitting node along same optical links, preferably in the form of same optical fibres and same intermediate network nodes, to a same destination node.
As it is evident from FIG. 1, a super-channel signal formed by a number of carrier signals C1, C2 may be given by a distribution of the carrier signals C1, C2 onto the slots S1, . . . , S12, such that the carrier signal C1 is distributed onto a number of slots S1, . . . , S4 that are not occupied by any other carrier signal C2. In other words, a single slot S1, . . . , S12 is occupied by at most one carrier signal.
Sticking to the grid shown in FIG. 1, the different carrier signals C1 and C2 may be received at a network node and may then also be equalized, e.g. attenuated, in their powers by an equalization method in which each slot equalization affects only one of the carrier signals.